|Fig. 1: The Arak Heavy Water Reactor in 2012 (Source: Wikimedia Commons)|
Referred to frequently as the "Iran Deal," the Joint Comprehensive Plan of Action (JCPOA), agreed to in 2015 and implemented on January 16, 2016, is a wide-ranging agreement between Iran and a coalition known as the E3/EU+3 (consisting of China, France, Germany, Russia, the United Kingdom, the United States, and the European Union) in order to ensure that the former's nuclear program may not be used to produce weaponry in the foreseeable future. 
While the end goal of the agreement is fairly simple (namely, to prevent Iran from acquiring a functional nuclear warhead), the provisions of the agreement are by necessity highly technical, and as a result, are generally not discussed in terms that are immediately accessible to the general reader. This article is intended to briefly summarize and explain these technical provisions in a manner accessible to general audiences, with no particular expertise in nuclear energy or nuclear physics assumed.
At its core, the nuclear provisions in the Iran deal attempt to limit Iran's ability to acquire and process so-called fissile material. Fissile material, in turn, refers to materials that can split in a nuclear fission reaction, releasing the energy which powers a nuclear reactor or weapon. In practice, fissile materials are usually specific isotopes of Uranium and Plutonium.
As the JCPOA's main provisions deal with the ability of the nation to acquire weapons-grade nuclear material, it is unsurprising that the agreement places strict limitations on Iran's access to Plutonium, the element upon which 95 percent of the world's nuclear warheads now rely. 
Much of Iran's so-called Plutonium route to a weapon centered on its heavy-water reactor in Arak, depicted in Fig. 1.  Heavy water is distinguished from normal (i.e., "light") water by the isotope of Hydrogen in the water molecule; in heavy water, the Hydrogen is Deuterium, which has a nucleus containing one proton and one neutron, while in light water, the more common Hydrogen isotope, with only a single proton and no neutron, is present.  Both heavy water and more common light water can act as a medium through which neutrons, produced by fission events, may travel to slow down to a speed at which they are likely to split another atomic nucleus in the fuel (in this capacity, the water is known as a moderator).  However, heavy water is far less likely to absorb neutrons propagating through it, meaning that in a heavy-water reactor far more neutrons are freely flying about the reactor core. 
In natural Uranium, 99.3% of a given sample is the isotope U-238, which does not generally split, while the fissile isotope, U-235, is significantly rarer, comprising approximately the other 0.7%. The excess neutrons in a heavy-water reactor allow natural Uranium (rather than Uranium that has been enriched to have a significantly higher percentage of U-235 than a natural sample would have) to be used as fuel. Equally importantly, excess neutrons absorbed by U-238 will transmute that element into the fissile isotope Plutonium-239.  As a result, a heavy-water reactor's spent fuel will include significant quantities of Plutonium.
Because this Plutonium is chemically distinct from Uranium, it is substantially simpler to isolate it from the other materials in spent nuclear fuel than it is to enrich Uranium to weapons-grade levels.  (Chemical separation techniques that distinguish between different chemical elements cannot be applied to separate different isotopes of the same element. ) In its original design, the Arak reactor, when completed, could have produced enough Plutonium for one or two weapons per year from natural (un-enriched) Uranium fuel, sparing its operators any need for a Uranium enrichment process to produce a weapon. [1,3] Given the destructive potential of a single nuclear warhead, this was cause for considerable international alarm.
Under the JCPOA, Iran has dismantled the original Arak reactor core, and filled its openings with concrete, rendering it inoperable.  In accordance with the deal, the Arak reactor is now being redesigned by an international consortium (including Iran and the E3/EU+3), so that it now requires enriched (as opposed to natural) Uranium fuel (up to 3.67% U-235 content, dramatically more than natural fuel, but dramatically less than the roughly 90% needed for weapons-grade Uranium). It will thus produce a minimal amount of usable Plutonium (made from the non-fissile U-238), and be dependent on Iran's Uranium enrichment infrastructure to function. [1,3]
Unlike the prior design, the Plutonium produced by the new Arak reactor will take several years to produce enough material for a single weapon. The isotopic mix of produced Plutonium will also not be weapons-grade, but instead only be capable of producing a crude, weaker weapon without the use of advanced technology that Iran is unlikely to possess. [3,5]
The JCPOA also contains provisions to limit Iran's ability to acquire a weapon through the Plutonium pathway, or acquire the infrastructure to do so, until at least the 2030's. First, the agreement bars Iran from building any additional heavy-water reactors for 15 years. (Iran also states its intention to abstain from building such reactors after 2031, however this commitment is not legally binding.) It also requires that Iran sell any reserves of excess heavy water beyond the amount needed to produce and operate the Arak reactor for the same time period (limiting Iran's ability to covertly construct an additional heavy water reactor), and that all of the spent fuel from the Arak reactor be shipped out of the country for the reactor's entire lifetime. [3,6] The agreement also stipulates that Iran will not conduct research on reprocessing technologies, or build any reprocessing facilities, until 2031, with the exception of hot cells with a volume of less than 6 cubic meters for producing medical isotopes.  For research purposes, Iran is not permitted to perform any chemical processing of the fuel within their country, instead being required to use facilities belonging to members of the E3/EU+3 should they require them. 
In practice, these provisions prevent Iran, which must subject its declared nuclear facilities to regular inspection under JCPOA provisions, from practically acquiring a ready source of weapons-grade Plutonium until at least 2031. [1,2,3,7] Because the Arak reactor (along with the rest of Iran's known nuclear facilities) is under careful monitoring as part of the JCPOA, it is highly unlikely that the reactor could be successfully modified prior to that year, or that the spent fuel produced by the reactor could plausibly be covertly kept for reprocessing. [1,3] Additionally, because Iran's heavy-water stockpile is continuously monitored under the agreement by the International Atomic Energy Agency (IAEA), and the import and export of nuclear reactor components in Iran must go through a newly-created "procurement channel," any Iranian attempts to create an additional heavy water reactor prior to 2031 will be extremely difficult. [1,3] Combined with the IAEA's power to inspect even undeclared sites that they suspect as being in violation of the JCPOA with limited notice, Iran's ability to covertly produce the large-scale facilities necessary for a realistic Plutonium pathway is essentially nonexistent until at least the 2030's. 
Unlike the Plutonium pathway with the Arak reactor (which was never completed before the signing of the Iran deal), Iran's Uranium-based pathway to a weapon involves a great deal of infrastructure that the nation has already developed.  In order to produce a weapon using Uranium, natural Uranium (with 0.7% U-235) must be enriched until it reaches weapons-grade (90% U-235). In order to enrich Uranium, centrifuges must be used, which take advantage of the slight difference in the mass of U-235 from the more common Uranium isotope U-238 to separate out the fissile isotope. Prior to the implementation of the Iran deal, Iran had nearly 20,000 centrifuges, of which approximately 10,000 were enriching Uranium, spread between two sites, the Natanz and Fordow Enrichment Plants. [1,3,8]
Using these facilities, Iran had accumulated a significant stockpile of enriched Uranium, consisting of approximately 10 tons of low-enriched Uranium (up to 3.67% U- 235), as well as a smaller stock of Uranium which possessed about 20% U-235 content. [1,3] The latter stockpile was particularly worrisome, because enriching natural Uranium to 20% U-235 constitutes approximately 90% of the effort required to produce weapons-grade Uranium.  Under the JCPOA, Iran has reduced its low-enriched Uranium (LEU) stockpile by approximately 98%, to only 300 kg of Uranium Hexafluoride gas and agreed to have their stockpile capped at that level for 15 years (for reference, the necessary material for a bomb, approximately 40 kg of weapons-grade Uranium Hexafluoride, would be approximately three times Iran's maximum allowed stockpile). [3,6,8]
Iran's number of active centrifuges has been dramatically reduced as well; under the agreement, enrichment was entirely halted at the Fordow facility, which will be converted into a nuclear technology and physics center, while only 5,060 centrifuges are allowed to stay active at Natanz for 10 years, in order to produce LEU of no greater than 3.67% U-235 (the cap on Uranium enrichment level is in place for 15 years). [1,6,8] Furthermore, Iran is forced to use only its older, less efficient centrifuge models at Natanz for the next 10 years, and may not produce new centrifuge units unless its reserve of existing surplus units (which may be substituted in to replace failed or damaged centrifuge units) falls below 500 during that time. [1,8] All of Iran's 20% U-235 fuel was either diluted to at most 3.67% U-235 or is to be used at a research reactor located in Tehran; the IAEA confirmed in January 2016 that all Tehran Research Reactor fuel plates have been irradiated, indicating that Iran's remaining stockpile of nearly weapons-grade fuel is being used for its stated purpose. [1,6]
In practice, this reduction in Iran's Uranium enrichment infrastructure substantially limits the nation's ability to produce Uranium-based weaponry for at least the next decade. If Iran were to spontaneously begin ignoring its limits on Uranium stockpiles (a move that would likely be detected by both the IAEA, which will keep the Natanz facility under continuous monitoring, and international intelligence agencies), the reduced number of centrifuges has lengthened the time in which Iran could produce enough weapons-grade Uranium for a bomb from 2 to 3 months to approximately a year.  Iran is also banned from research and development work on improving their centrifuge technology for the next decade, and therefore if it wishes to improve its centrifuge technology before then, it must do so covertly and likely without easily detected fissile material. [1,3]
For the next 10 to 15 years, the JCPOA makes the production of a fully functional nuclear weapon by Iran at worst highly impractical and at best a virtual impossibility. While the agreement does cease to enforce a significant number of its provisions after this time frame (notably, after 2026, Iran may begin employing more efficient centrifuges, while after 2031 there are virtually no legally binding restrictions on Uranium enrichment or Plutonium production), it is difficult to envision a scenario where Iran is capable of engineering a covert supply chain of fissile material.  As a result, due to the restrictions specifically on Iran's ability to acquire fissile materials, the Iran nuclear deal has effectively frozen the nation's nuclear weapons capacity for at least the next decade.
© George Wojcik. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 K. Katzman and P.K. Kerr, "Iran Nuclear Agreement," Congressional Research Service, R43333, September 2017.
 W. J. Broad, "Plutonium Is Unsung Concession in Iran Nuclear Deal," New York Times, 7 Sep 15.
 N. Burns et al., "The Iran Nuclear Deal: A Definitive Guide," Belfer Center for Science and International Affairs, Harvard kennedy Schol, August 2015.
 M. Fisher "A Nuclear Expert Explains, in Very Basic Language, the Science at the Heart of the Iranian Nuclear Talks," Washington Post, 12 Nov 13.
 "Fissile Materials Basics," Union of Concerned Scientists, April 2004.
 "Verification and Monitoring in the Islamic Republic of Iran in Light of United Nations Security Council Resolution 2231 (2015)," International Atomic Energy Agency, GOV/INF/2016/1, 16 Jan 16.
 "The Plutonium Pathway: Arak Heavy Water Reactor and Reprocessing," Institute for Science and International Security, 21 Jul 15.
 P. Saffari, "The Iran Nuclear Deal," Physics 241, Stanford University, Winter 2018.